Geologic plates pressing against each other slipped violently, creating a bulge on the bottom that could be as high as 10m and hundreds of kilometres long, one scientist said.
"It's just like moving an enormous paddle at the bottom of the sea," said David Booth, a seismologist at the British Geological Survey. "A big column of water has moved, we're talking about billions of tons. This is an enormous disturbance."
Moving at about 800kph the waves took more than two hours to reach Sri Lanka, where the human toll has been horrific, and longer to spread to India and the east coast of Africa.
And because such tidal waves rarely occur in the Indian Ocean, there is no system in place to warn coastal communities they are about to be hit, such as exists in the Pacific, Booth said.
"With 20-20 vision of hindsight, that will be reconsidered," he said.
An Australian scientist had suggested in September that an Indian Ocean warning system be set up, but it takes a year to create one.
Also, those living along the Indian Ocean's shores were less likely than Pacific coastal dwellers to know the warning signs of an impending tidal wave - water receding unusually fast and far from the shore, Booth said.
Thousands were killed in countries from Indonesia to Somalia.
The underwater quake, which the US Geological Survey put at magnitude 9, was the biggest since 1964, when a 9.2-magnitude tremor struck Alaska, also touching off tsunami waves.
There were at least a half-dozen powerful aftershocks, one of magnitude 7.3.
Million atomic bombs
Enzo Boschi, the head of Italy's National Geophysics Institute, likened the quake's power to detonating one million atomic bombs the size of those dropped on Japan during the second world war, and said the shaking was so powerful it even disturbed the Earth's rotation.
"All the planet is vibrating [from the quake]"
Enzo Boschi, head of Italy's National Geophysics Institute
"All the planet is vibrating" from the quake, he told Italian state radio. Other scientists said it was too early to say whether the rotation was affected by the quake.
The earthquake occurred at a spot where the Indian Ocean plate is gradually being forced underneath Sumatra, which is part of the Eurasian plate, at about the speed at which a human fingernail grows, Booth explained.
"This slipping does not occur smoothly," he said. Rocks along the edge stick against one another and pent-up energy builds over hundreds of years.
It is "almost like stretching an elastic band, and then when the strength of the rock is not sufficient to withstand the stress, then all along the fault line the rocks will move," he said.
is well-known as a major quake centre, sitting along a series of fault lines dubbed the Ring of Fire. But scientists are unable to predict where and when quakes will strike with any precision.
Ring of Fire
The force of Sunday's earthquake shook unusually far afield, causing buildings to sway hundreds of kilometres from the epicentre, from Singapore to Chiang Mai in northern Thailand to Bangladesh.
The force of the quake could be
felt far and wide
The quake probably occurred about 10km beneath the ocean floor, causing the huge, step-like protrusion on the sea bed and the resulting tidal waves.
As the waves moved across deep areas of the ocean in the early morning, they may have been almost undetectable on the surface, with swells of about a metre or less.
But when they approached land, the huge volumes of water were forced to the surface and the waves grew higher, swamping coastal communities and causing massive numbers of casualties.